Bizarre keyboard behaviour after 4.2 upgrade

Question

Since upgrading to KDE 4.2 (from 4.1) as part of a system upgrade, my system has been practically unusable due to the bizarre keyboard behaviour within KDE.
Right from the KDM login screen, most of the keys do not work as normal.  For most letters, the key has to be pressed twice in order to generate the letter.  However, I cannot type anything with a "u" because it is interpreted as a shortcut to undo everything I have just entered!  Letter "f" has to be typed 4 times.  If I type "h" once it appears; if I type it again it disappears.  Letter "l" produces a vertical division in the window.  I could go on....
Since my password ends in "u" I even had to change the password in order to login to KDE because the "u" would delete the characters I'd already entered!  But once logged in the problem is just the same.
I have tried setting a different keyboard layout within KDE but this made no difference.
I have tried uninstalling and reinstalling KDE.
The problem seems to be KDE related, because outside KDE, in a system console, the keyboard works absolutely normally.  And it doesn't seem to be related to the individual KDE user, since as I said, the problem is present as soon as the KDM login screen appears.
Has anyone got any ideas please?

Answer 1

Thanks for the suggestions.  Although they did not prove to be the answer, I have now overcome the problem.  I suspect it was a one-off caused by corrupted data, probably in the xorg installation rather than KDE.  I removed xorg then reinstalled it - some of the pkg files were corrupt and had to be replaced.  Then I had trouble starting the X server at all, and had to reinstall several library packages.  The system is running more normally now, though I think there are still some data corruptions in there and time will tell whether the installation is salvageable.